Synthesis, capping and dispersion of nanocrystals

ABSTRACT

Preparation of semiconductor nanocrystals and their dispersions in solvents and other media is described. The nanocrystals described herein have small (1-10 nm) particle size with minimal aggregation and can be synthesized with high yield. The capping agents on the as-synthesized nanocrystals as well as nanocrystals which have undergone cap exchange reactions result in the formation of stable suspensions in polar and nonpolar solvents which may then result in the formation of high quality nanocomposite films.

This application is a continuation of U.S. application Ser. No.13/064,905 (allowed), filed Apr. 25, 2011 (published asUS-2012-0088845-A1), which claims benefit of U.S. ProvisionalApplication Nos. 61/327,313 and 61/407,063, filed Apr. 23, 2010 and Oct.27, 2010, the entire contents of each of which is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This work was supported in part by Department of Commerce CooperativeAgreement Nos. 70NANB7H7014 and 70NANB10H012 and the National ScienceFoundation grant no. 0724417.

TECHNICAL FIELD

Preparation of capped colloidal semiconductor nanocrystals and theirdispersions in polymeric solutions and films are described herein. Thecolloidal semiconductor nanocrystals are highly monodisperse withnanocrystal size between 1-10 nm. Nanocomposites having a high loadingdensity of uniformly dispersed capped semiconductor nanocrystalsincorporated therein may be formed with these nanocrystals. Suspensionsof nanocrystals may be formed in various solvents and nanocompositesmade of same may be made optically transparent with very little or noscattering.

Nanocrystals are single crystals of a material in which at least onedimension of the crystal is less than 1 micron in size. Nanocrystals mayeither retain the optical, mechanical, and physical properties of theirbulk counterparts or display properties which are quite different.Nanocrystals can be made by a wide variety of methods, some of whichinclude: liquid synthesis, solvothermal synthesis, vapor phasesynthesis, aerosol synthesis, pyrolysis, flame pyrolysis, laserpyrolysis, ball-milling, and laser ablation.

Nanocrystals can be dispersed into a variety of media or combination ofmedia, including, but not limited to: liquids, gels, glasses, polymersand solids. The dispersed nanocrystals may impart all or some of theproperties of the nanocrystals upon the dispersion or may give thedispersion properties which are different from any of the individualcomponents. The quality of the dispersion created between thenanocrystals and the media can have a large effect on the properties ofthe final dispersion. The quality of the dispersion of the nanocrystalsin a medium can be described as being governed by complex interactionsbetween a set of parameters, which include, but are not limited to: thechemistry of the nanocrystal surface (or the effective nanocrystalsurface), the size and shape of the nanocrystals, the physicalproperties of the nanocrystals, the chemistry of the dispersion media,and the physical properties of the dispersion media. Well-dispersednanocrystals can be defined as nanocrystals which are uniformlydistributed throughout the media with a minimal amount of nanocrystalaggregates present. If the nanocrystals are not well-dispersed in themedium, the optical, mechanical, and physical properties of thenanocrystals may be altered or the properties of the media may beadversely affected.

Nanocomposites are nanocrystal dispersions composed of nanocrystalsdispersed in a matrix including: polymers, ceramics and glasses.Nanocomposites can be made by the mixing of nanocrystals, either inpowder form or already dispersed in another media, with precursorcomponents of the matrix. A non-exhaustive list of potential matrixcomponents for use in the formation of nanocomposites includes:monomers, oligomers, polymers, pre-polymeric resins, ceramics,pre-ceramics and glasses. Nanocomposites can be considered to be anextension of the well-known field of composites, where the micron-sized,or larger, fillers used in composites have been replaced bynanocrystals. In both composites and nanocomposites it may be possibleto modify the optical, mechanical, and physical properties of thenanocomposites with the filler materials, but the reduced size of thefillers used in nanocomposites may result in relatively fewer, or lessintense, detrimental effects due to the inclusion of a filler into thematrix. A list of these potentially detrimental effects which may happento the composite include: reduced structural integrity, reducedmechanical strength, reduced mechanical stability, reduced flexibility,reduced optical transparency, and reduced thermal stability. To morefully realize the potential of using nanocrystals as replacements formicron size, or larger, fillers, the nanocrystals need to be able to bewell-dispersed in the matrix. This is due to the fact that aggregatednanocrystals in the composite act as detrimentally as, or worse than,fillers of the size of the aggregates. Thus a composite made of heavilyaggregated 5 nm particles, where the size of the aggregates are greaterthan 1 micron in all dimensions may not behave as a nanocomposite.

Typical routes for the manufacture of nanocomposites often result in adistribution of nanocrystals in the media that cannot be described aswell-dispersed. The distribution of the nanocrystals is oftennon-uniform and contains large amounts of aggregates. One key toproducing well-dispersed nanocomposites is to use nanocrystals which arenot aggregated before the start of mixing with the matrix or media.

There are two main types of aggregates that are often discussed inliterature. Hard aggregates are clusters of nanocrystals, in which thenanocrystals are relatively strongly bound to each other. Hardaggregates may be the result of particles that have come into contactduring formation or after formation but while the materials are still atelevated temperatures. The other type of aggregates, soft aggregates, isusually formed after synthesis, or at lower temperatures. Theconventional wisdom is that soft aggregates can be broken apart easilyduring processing and can thus be made to be well-dispersed, whereashard aggregates cannot be broken apart without great difficulty andtherefore are not suitable sources of well-dispersed nanocrystals. Inorder to form dispersions in which the nanocrystals are well dispersed,it is preferable to avoid both types of aggregation.

Nanocrystal aggregation is controlled by the surface chemistry (orchemistry of the effective surface) of the nanocrystals. In adispersion, the inter-particle forces (such as electrostatic forces, vander Waals forces and entropic forces) between the surfaces of thenanocrystals result in a tendency to form aggregates. Theseinter-particle forces are particularly important in nanocrystals becauseof the large surface to volume ratio for these particles. In order toavoid aggregation in dispersion it is desirable for the surfaces of thenanocrystals to be passivated (or stabilized). One method that may beused to passivate the surface of the nanocrystal involves theintroduction of ligand ions or molecules. These ligands, which are alsocalled capping agents or caps, are added to the surface of thenanocrystals and thus create a new effective surface of thenanocrystals. This effective surface is the surface of the shell createdby the complete or partial surface coverage with ligands. The chemistryof this effective surface can be tailored in order to create a chemicalenvironment, distinct from the actual or initial surface of thenanocrystal, which facilitates dispersion while preventing or reducingaggregation. These passivating ligands can help prevent aggregation in avariety of ways. Electrostatic passivation, utilizing like charges torepulse the nanocrystals, and steric passivation, using bulky moleculesto physically keep the nanocrystal surfaces apart, are two examples ofsurface passivation methods.

Most typical nanocrystal synthetic methods, such as aerosol synthesis,pyrolysis, flame pyrolysis, laser pyrolysis, ball-milling, and laserablation, produce nanocrystals which have no surface passivation of thetypes described herein. In fact, many of these methods producenanocrystals that are clustered together as hard aggregates. Even if thesynthesis does not result in aggregated nanocrystals, metal oxidenanocrystals without surface passivation tend toward aggregation becauseof inter-particle forces.

The liquid synthesis of metal oxide colloidal nanocrystals is a methodof producing nanocrystals which are, at least partially, surfacepassivated during the synthesis. The liquid synthesis is performed insolvent with or without the presence of capping agents. The nanocrystalsare protected against aggregation, at least partially, during thesynthesis and afterwards, by capping agents. In cases where thesynthesis is carried out in a coordinating solvent, the solventmolecules, or products thereof, may act as the capping agent topassivate the surface. After liquid synthesis, the nanocrystals areprotected from forming aggregates by partial or total coverage of thenanocrystals with solvent(s), product(s) of the solvent(s), addedcapping agent(s), and/or combination thereof.

After synthesis of nanocrystals by liquid synthesis, the as-made surfacepassivation can be modified by a process known as a cap exchange orligand exchange reaction in which one ligand or capping agent is atleast partially replaced by a different one. In this process thenanocrystals are usually dispersed in a solvent along with the desiredcapping agent. In some instances the temperature of the suspension maybe elevated to further drive the exchange process. As a result of thecap exchange, either the new capping agent is added to some fraction ofthe nanocrystal surface or a fraction of the previous surfacepassivation agents are replaced by the new capping agent, or somecombination thereof. The new capping agent may be chosen in order toyield chemical compatibility between the effective nanocrystal surfaceand the solvent, or other media, chosen for the final dispersion orapplication.

As-synthesized nanocrystals, which have been produced by other methodsand do not have surface passivation, can also be exposed to cappingagents. While this also may result in some fraction of the surface ofthe nanocrystals being covered by the capping agents, this process maynot be able to break apart any aggregates which will have formedpreviously, including both hard and soft aggregates. These aggregates ofoxide nanocrystals are distinct from very weakly bound agglomerates ofsurface passivated nanocrystals where the passivation agents may createa porous spacer between the nanocrystals. In the weakly boundagglomerates, the inter-nanocrystal spacer layers provided by surfacepassivation are important because many of the surface to surface forceswhich cause aggregation are short range interactions, which can bereduced by the increased nanocrystal separation. However, in the absenceof surface passivation, once the nanocrystal surfaces have been broughttogether, such as in the formation of hard aggregates, the short rangeforces dominate and it is difficult to separate the nanocrystals again.

Agglomerates of surface passivated nanocrystals, which can be broken up,may form during various points in the production of a dispersion,including during the washing of the particles, and the drying ofpowders. One of the advantages of using liquid synthesis to producecolloidal nanocrystals is that surface passivation of the as-synthesizednanocrystals can be used to prevent or reduce both hard and softaggregates from forming during all stages of nanocrystal processing fromthe synthesis, to post-synthetic processing, to formation of the finalhigh quality dispersion.

SUMMARY

In order to achieve higher quality nanocomposites, nanocrystal particlesize should advantageously be less than 10 nm in at least one dimension,with preferably a very narrow particle size distribution, and furtherwith specific particle shape (rod, spherical, etc). In addition, thesurface chemistry of the nanocrystal is advantageously well passivated,preventing or reducing aggregation, and increasing or enhancingcompatibility with the solvent(s) and/or the matrix material, andthereby allowing or enhancing dispersion of the nanocrystals into ananocomposite or other substrate containing same.

Nanocrystals of the present disclosure will also be recognized in theart as including, for example, nanoparticles, quantum dots and colloidalparticles and can include particles that are crystalline and/oramorphous with sizes ranging from a few hundred nanometers down to 1 nmor less. Due to their small size, nanocrystals can possess dramaticallydifferent physical properties compared to bulk forms of similarmaterials, due, for example, to the quantum effect and/or a greaterarea/volume ratio. Nanocrystals of the present disclosure may be usefulin, for example, applications ranging from metallurgy to chemicalsensors, and industries ranging from pharmaceuticals to paints andcoatings to cosmetics. Microelectronic and optical applications are alsocontemplated.

Colloidal semiconductor nanocrystals are chemically synthesized, on thenanometer scale with ligands or capping agents on the surface of thenanocrystals to afford both dispersibility and stability in solution. Ina basic chemical synthetic route, the precursors of the semiconductornanocrystals react or decompose in the presence of a stabilizing organiccapping agent or a solvent. Varying the size of the nanocrystals can beachieved by changing the reaction time or temperature profile, oradjusting the sequence of precursor addition, or varying theconcentrations of chemical precursors, or varying the ratios ofconcentrations of chemical precursors, and/or varying the cappingagents.

The chemistry of the capping agent effects and/or controls several ofthe system parameters in the manufacture of the nanocrystals and/or thenanocomposites, such as the growth rate, shape, and dispersibility ofthe nanocrystals in a variety of solvents and solids, and even theexcited state lifetimes of charge carriers in the nanocrystals. Theflexibility of the resulting effects of this chemical synthesis isdemonstrated by the fact that often one capping agent is chosen for itsgrowth control properties and is later substituted out, either partiallyor fully, after synthesis for a different capping agent. Thissubstitution may be carried out for a variety of reasons, including, butnot limited to: in order to provide a nanocrystal/media interface moresuitable to the given application or to modify the optical properties ofthe nanocrystal.

Synthetic methods for producing colloidal semiconductor nanocrystals ofzinc oxide (ZnO), yttrium oxide (Y₂O₃), hafnium oxide (HfO₂), andzirconium oxide (ZrO₂), hafnium-zirconium oxide [HfO₂:ZrO₂] andtitanium-zirconium oxide [TiO₂:ZrO₂], as well as capping andcap-exchange of these nanocrystals and dispersion of these materials insolvents and polymers and the creation of nanocomposites are describedherein.

Functionalized organosilanes are a common class of organic compoundsused to populate the surface of a nanocrystalline oxide material ascapping agents. These organosilanes are typically composed of head andtail components. The head of a functionalized organosilane is typicallyeither a trialkoxysilane group or a trichlorosilane group, although bi-and mono-substituted alkoxy and chloro silane are possible. The headanchors to the surface of the oxide through a covalent bond with thehydroxide groups (—OH) or —OR group wherein R is an alkyl or aryl group,present at the surface, eliminating an alcohol, alkyl chloride, water orHCl as a by-product. The tails of a functionalized organosilane caninclude one, or more of an alkyl chains of varying lengths, aryl groups,or ether groups, amines, thiols, or carboxylic acid.

FIG. 1 show an exemplary attachment of an organosilane to a nanocrystalsurface through an alcohol elimination reaction. In this reaction, thenanocrystals with a polar surface containing —OH groups (101) react withan organosilane (102) to form the organosilane capped nanocrystals(103).

Other classes of organic compounds used as capping agents to passivatethe surface of an oxide material include organocarboxylic acids andorganoalcohols. The head of organocarboxylic acids is a carboxylic acid(—COOH) group and organoalcohols is an —OH group. The head anchors tothe surface of the oxide through a covalent bond with the hydroxidegroups (—OH) or —OR(R=alkyl or aryl) group present at the surface,eliminating an alcohol, or water as a by-product. The tails of afunctionalized organocarboxylic acids and organoalcohols can be composedof alkyl chains of a variety of lengths, aryl groups, ether groups,amines, thiols, or carboxylic acids.

The use of a capping agent such as functionalized organosilanes,alcohols or carboxylic acids on colloidal nanocrystals impart a numberof desired characteristics, such as for example, controlling theircompatibility to various dispersing solvents, such as polar or non-polarmedia, which can thereby reduce nanocrystal aggregation.

The present disclosure further includes methods for the surfacemodification of nanocrystals with organosilanes, organoalcohols and/ororganocarboxylic acids. The method includes depositing capping agentsduring the synthesis of the nanocrystals or through ligand exchange ofat least part of the capping agent originally present on the nanocrystalwith a second one after the synthesis. These reactions can be performedunder ambient, heated, and/or high temperature/high pressure conditions.

The present disclosure further includes a nanocomposite materialcontaining a matrix and nanocrystals, which have been, for example,mixed, stirred, or dispersed therein. Nanocomposites according to thepresent disclosure may be fabricated by, for example, melt blending, insitu polymerization, and/or solvent mixing of the nanocrystals and thematrix materials or precursors of the matrix.

In melt blending, nanocrystals are mixed with a polymer in its moltenstate with the assistance of mechanical forces. In situ polymerizationinvolves mixing nanocrystals with monomer(s) which are then polymerizedto form a composite. Solvent mixing involves the use of solvent(s) todisperse both the nanocrystals and the polymer whereby uniformdispersion of the polymer and nanocrystals is achieved by removal of thesolvent.

The present disclosure includes preparation methods for nanocompositematerials which include solvent mixing of polymers, or polymerprecursors, with nanocrystals capped with a functionalizedorganosilanes, organoacids or organoalcohols; and in situ polymerizationof capped nanocystals and monomers of polymers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the attachment of an organosilane to a nanocrystalsurface through alcohol elimination

FIG. 2 shows, in a block diagram, process steps of the presentdisclosure for the formation of a nanocomposite material describedherein.

FIG. 3 exemplifies a silane capped colloidal semiconductor nanocrystalsin a polymeric film.

FIG. 4 shows the TEM image of nanocrystals synthesized from zirconiumbutoxide.

FIG. 5 shows the XRD patterns of ZrO₂ nanocrystals synthesized fromzirconium propoxide by removing 1-propanol before, during or after thereaction.

FIG. 6 shows the TEM images of ZrO₂ nanocrystals synthesized fromzirconium propoxide by removing 1-propanol a) before, and b) after thereaction.

FIG. 7 shows TEM images of ZrO₂ nanocrystals with different sizes

FIG. 8 shows the TEM image of the as-synthesized HfO₂ nanocrystals withrice-like morphology.

FIG. 9 shows a TEM image of the 2-5 nm HfO₂ nanocrystal.

FIG. 10 shows a TEM image of ZnO nanocrystals.

FIG. 11 shows the UV-Vis spectra of a ZnO/SOG nanocomposite spin coatedfilms

FIG. 12 shows the TEM images of a ZnO/PMMA nanocomposite

FIG. 13 shows the TEM images of a HfO₂/SOG nanocomposite

FIG. 14 shows surface roughness of a ZrO₂ film as measured by AFM.

DETAILED DESCRIPTION

The synthetic methods to prepare high quality semiconductor metal oxidenanocrystals described herein include synthetic methods wherein aprecursor of the metal oxide is mixed or dissolved in at least onesolvent and allowed to react for a certain period of time. The use ofpressure or heating may be necessary in some cases.

At least in the case of ZrO₂ and HfO₂ nanocrystal syntheses, addition ofwater into the solvent surprisingly results in smaller particles thanreactions carried out without addition of water, as described in theexamples. By controlling the amount of water added to the solvent theaverage particle size of the nanocrystals can be controlled.

The precursors of the metal oxides may be one or more of alkoxides, suchas: zirconium ethoxide (Zr(OCH₂CH₃)₄), zirconium n-propoxide(Zr(OCH₂CH₂CH₃)₄), zirconium isopropoxide (Zr(OCH(CH₃)₂)₄), zirconiumn-butoxide (Zr(OCH₂CH₂CH₂CH₃)₄), zirconium t-butoxide (Zr(OC(CH₃)₃)₄),hafnium ethoxide (Hf(OCH₂CH₃)₄), hafnium n-propoxide (Hf(OCH₂CH₂CH₃)₄),hafnium isopropoxide (Hf(OCH(CH₃)₂)₄), hafnium butoxide(Hf(OCH₂CH₂CH₂CH₃)₄), hafnium t-butoxide (Hf(OC(CH₃)₃)₄), titaniumethoxide (Ti(OCH₂CH₃)₄), titanium n-propoxide (Ti(OCH₂CH₂CH₃)₄),titanium isopropoxide (Ti(OCH(CH₃)₂)₄), titanium t-butoxide(Zr(OC(CH₃)₃)₄), titanium n-butoxide (Ti(OCH₂CH₂CH₂CH₃)₄), zinc ethoxide(Zn(OCH₂CH₃)₂), zinc n-propoxide (Zn(OCH₂CH₂CH₃)₂), zinc isopropoxide(Zr(OCH(CH₃)₂)₂), zinc butoxide (Zn(OCH₂CH₂CH₂CH₃)₂); acetates oracetylacetonates, such as, zirconium acetate (Zr(OOCCH₃)₄), zirconiumacetylacetonate (Zr(CH₃COCHCOCH₃)₄), zinc acetate (Zn(OOCCH₃)₂), zincacetylacetonate (Zn(CH₃COCHCOCH₃)₂), hafnium acetate (Hf(OOCCH₃)₄);halides such as zirconium chloride (ZrCl₄), zirconium fluoride (ZrF₄),zirconium iodide (ZrI₄), zirconium bromide (ZrBr₄), hafnium bromide(HfBr₄), hafnium chloride (HfCl₄), hafnium iodide (HfI₄), titaniumchloride (TiCl₄), titanium bromide (TiBr₄), titanium iodide (TiI₄),titanium fluoride (TiF₄), zinc chloride (ZnCl₂), zinc bromide (ZnBr₂),zinc iodide (ZnI₂), zinc fluoride (ZnF₂) or other organometalliccompounds.

Solvents useful in the present disclosure include benzyl alcohol,phenol, oleyl alcohol, toluene, butanol, propanol, isopropanol, ethanol,water, propylene glycol monomethyl ether (PGME), propylene glycol methylether acetate (PGMEA), ethyl lactate (EL), and 2-propoxy-propanol (PnP),acetone, tetrahydrofuran, cyclic ketones and mixtures thereof.

The surface of nanocrystals of the present disclosure are optionallycapped with at least one capping agent such as organosilane,organoalcohol or organocarboxylic acid. Examples of organosilanes of thepresent disclosure include, n-propyltrimethoxysilane,n-propyltriethoxysilane, n-octyltrimethoxysilane,n-octyltriethoxysilane, phenylrimethoxysilane,2-[methoxy(polyethyleneoxy)propyl]-trimethoxysilane,methoxy(triethyleneoxy)propyltrimethoxysilane,3-aminopropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane,3-(methacryloyloxy)propyl trimethoxysilane,3-isocyanatopropyltriethoxysilane, 3-isocyanatopropyltrimethoxysilane,and glycidoxypropyltrimethoxysilane.

Examples of organoalcohols of the present disclosure include, heptanol,hexanol, octanol, benzyl alcohol, phenol, ethanol, propanol, butanol,oleylalcohol, dodecylalcohol, octadecanol and triethylene glycolmonomethyl ether.

Examples of organocarboxylic acids of the present disclosure include,octanoic acid, acetic acid, propionic acid,2-[2-(2-methoxyethoxy)ethoxy]acetic acid, oleic acid, benzoic acid.

Capped colloidal semiconductor nanocrystals of the present disclosureare, optionally, removed from and re-dispersed into solvents, such as,water, tetrahydrofuran, ethanol, methanol, acetonitrile, PGMEA, PGPE,PGME, cyclic ketones, ethyl lactate, acetone, naphtha, hexane, heptane,toluene or a mixture thereof.

Semiconductor nanocrystals can be added into a matrix to form ananocomposite. The matrix material of the present disclosure include,poly(acrylonitrile-butadiene-styrene) (ABS), poly(methyl methacrylate)(PMMA), celluloid, cellulose acetate, poly(ethylene-vinyl acetate)(EVA), poly(ethylene vinyl alcohol) (EVOH), fluoroplastics,polyacrylates (Acrylic), polyacrylonitrile (PAN), polyamide (PA orNylon), polyamide-imide (PAI), polyaryletherketone (PAEK), polybutadiene(PBD), polybutylene (PB), polybutylene terephthalate (PBT),polycaprolactone (PCL), polychlorotrifluoroethylene (PCTFE),polyethylene terephthalate (PET), polycyclohexylene dimethyleneterephthalate (PCT), polycarbonate (PC), polyhydroxyalkanoates (PHAs),polyketone (PK), polyester, polyethylene (PE), polyetheretherketone(PEEK), polyetherketoneketone (PEKK), polyetherimide (PEI),polyethersulfone (PES), polyethylenechlorinates (PEC), polyimide (PI),polylactic acid (PLA), polymethylpentene (PMP), polyphenylene oxide(PPO), polyphenylene sulfide (PPS), polyphthalamide (PPA), polypropylene(PP), polystyrene (PS), polysulfone (PSU), polytrimethyleneterephthalate (PTT), polyurethane (PU), polyvinyl acetate (PVA),polyvinyl chloride (PVC), polyvinylidene chloride (PVDC),poly(styrene-acrylonitrile) (SAN); a spin-on-glass (SOG) polymer, suchas: Siloxane-spin-on polymers in Ethanol, Propylene Glycol Methyl EtherAcetate (PGMEA), isopropyl alcohol or mixture of these solvents, JSRMicro topcoat (NFC TCX 014 in 4-methyl-2-pentanol), JSR Microphotoresist (ARF 1682J-19), and silicones, such as: Polydimethylsiloxane(PDMS) and polymethylphenylsiloxane.

Examples of nanocrystals include, but are not limited to, CuCl, CuBr,CuI, AgCl, AgBr, AgI, Ag₂S, Ag₂Te, Al₂O₃, Ga₂O₃, In₂O₃, FeO, Fe₂O₃,Fe₃O₄, TiO₂, MgO, Eu₂O₃, CrO₂, CaO, MgO, ZnO, Mg_(x)Zn_(1-x)O, SiO₂,Cu₂O, Zr₂O₃, ZrO₂, SnO₂, ZnS, HgS, Fe₂S, Cu₂S, CuIn₂S₂, MoS₂, In₂S₃,Bi₂S₃, GaP, GaAs, GaSb, InP, InAs, In_(x)Ga_(1-x)As, SiC,Si_(1-x)Ge_(x), CaF₂, YF₃, YSi₂, GaInP₂, Cd₃P₂, CuIn₂Se₂, In₂Se₃, HgI₂,PbI₂, ZnSe, CdS, CdSe, CdTe, HgTe, CdHgTe, PbS, BN, AlN, GaN, InN,Al_(x)Ga_(1-x)N, Si₃N₄, ZrN, Y₂O₃, HfO₂, Sc₂O₃, and their mixtures oralloys, wherein x may have a value of between 0.01 to 0.99.

The present disclosure provides a method of making nanocrystalsincluding dissolving precursors of said nanocrystals in at least onesolvent to produce a solution, optionally at least one of heating andincreasing pressure of said solution, and reacting the precursors or theprecursors and the at least one solvent of the solution to form thenanocrystals.

The nanocrystals may be capped with at least one agent to increase thesolubility or dispersibility of the nanocrystals in at least one solventor other media, or some combination of solvent and other media.

In the method of the disclosure, nanocrystals may be capped with atleast one agent which may include at least one organosilane,organoalcohol or organocarboxylic acid. These capping agents may impartuniform dispersion of the nanocrystals in different media such ashydrophobic or hydrophilic media by creating an effective nanocrystalsurface which is formed by the full or partial shell of capping agentswhose tail groups have a polarity compatible with the media.

The capping method of the present disclosure may including capping ofthe nanocrystals with the at least one capping agent in the solution,prior to, during, or after said reacting the precursors. The methods ofthe present disclosure further include purifying and/or separating thenanocrystals prior to, or after, the capping method of the presentdisclosure.

The method of the disclosure includes capping the as-synthesized,purified, and/or separated nanocrystals with at least one capping agentto produce at least partially capped nanocrystals. The at leastpartially purified capped nanocrystals may be further purified and/orseparated according to methods of the present disclosure. Nanocrystalsand capped nanocrystals may be dispersed in a material, includingsolvent, polymer, or some combination thereof in methods of the presentdisclosure. FIG. 2 is a block diagram exemplifying formation of acolloidal suspension. In the exemplified method, nanocrystals aresynthesized (101), capped or cap exchanged with at least one cappingagent (102), purified (103) and mixed with solvents or polymer solutions(104).

The present disclosure further includes methods of exchanging, fully orpartially, the pre-existing organic moieties or other capping agentspresent on the nanocrystal surface resulting from the synthesis of thenanocrystals or other previous cap exchange reactions withfunctionalized organosilanes, organoalcohols and organocarboxylic acidsin a cap exchange reaction.

Functionalized capping agents are covalently bonded to colloidalsemiconductor nanocrystals according to an aspect of the presentdisclosure during synthesis of the colloidal semiconductor nanocrystal.

Functionalized capping agents are optionally covalently bonded tosemiconductors in the present disclosure by removing pre-existingorganic moieties from the surface of semiconductor nanocrystals with anacid and then covalently bonding the functionalized capping agents tothe surface of the semiconductor nanocrystals. Examples of acids toremove pre-existing organic moieties include, for example, strong acids(e.g., HCl, HNO₃, and/or H₂SO₄), weak acids (e.g., H₃PO₄), and/ororganic acids (e.g., acetic acid).

Alternatively, nanocrystals are functionalized with capping agentswithout forming covalent bonds.

The present disclosure includes nanocrystals and at least partiallycapped nanocrystals made by methods described herein.

Methods of the present disclosure further includes methods of forming afilm or coating including dispersing the nanocrystals or at leastpartially capped nanocrystals of the present disclosure in a furthermaterial to form a dispersion, and applying the dispersion to a surface.The applying methods may include spin coating, spraying, dipping, screenprinting, rolling, painting, printing, ink jet printing, depositing byevaporation and/or vapor deposition.

Methods of the present disclosure include forming a nanocomposite whichincludes combining the nanocrystals or the at least partially cappednanocrystals of the present disclosure with a further material andforming the nanocomposite. FIG. 3 is an exemplary picture ofnanocrystals (102) capped with a capping agent (101) dispersed in apolymer matrix (103).

Methods of the present disclosure include forming a nanocomposite by,for example, curing, polymerization, laminating, extrusion, injectionmolding, mold casting, spin coating, dip coating, brushing, spraying,and/or printing.

The present disclosure further includes methods of forming homogeneousmixtures of the components of a nanocomposite with a variety ofdifferent methods, before assembling the components into the finalcomposite material suitable for desired applications.

The nanocrystals or at least partially capped nanocrystals of thepresent disclosure may be formed from zinc oxide, hafnium oxide,zirconium oxide, titanium-zirconium oxide, hafnium-zirconium oxide,yttrium oxide or other semiconductor material.

The products and methods of the present disclosure are exemplified bythe following non-limiting examples.

Example 1 Synthesis and Capping of Nanocrystals

Synthesis of Zirconium Oxide (ZrO₂) Nanocrystals

Zirconium oxide nanocrystals having a size in the range of 1-10 nm canbe prepared from precursors such as Zirconium (IV) n-butoxide, zirconiumn-propoxide, Zirconium isopropoxide isopropanol or zirconium ethoxide.Zirconium n-butoxide or zirconium n-propoxide would be advantageouslyused as precursors depending on final product desired.

In an exemplary method, a zirconium alkoxide precursor, such as, but notlimited to, Zirconium n-butoxide, zirconium n-propoxide, zirconiumisopropoxide isopropanol or zirconium ethoxide, is mixed with a solventor mixture of solvents, including benzyl alcohol, phenol, oleyl alcohol,butanol, propanol, isopropanol, water, tetrahydrofuran, ethanol,methanol, acetonitrile, toluene, PGMEA, PGPE, PGME, 2-methyl-1-propanol,or triethylene glycol monomethyl ether and sealed within an autoclave.The reaction mixture is heated to a temperature between 250-350° C. Oncethe reaction mixture reaches the set temperature, the temperature ismaintained for a length of time ranging from 20 minutes to 24 hoursdepending in part on the solvent or solvent mixtures and/or thetemperature of the reaction. As-synthesized zirconium oxide nanocrystalsare collected as a white milky suspension.

In a further example, zirconium oxide nanocrystals were produced from amixture of 30 millimoles of zirconium isopropoxide isopropanol orzirconium ethoxide and 300 milliliters of benzyl alcohol in an inertatmosphere which was sealed within an autoclave. The reaction mixturewas heated to 350° C. at a heating rate is 10° C./min. Once the reactionmixture reached 350° C., the temperature was maintained for 20-60 min. Awhite milky solution of as-synthesized ZrO₂ nanocrystals was collectedafter the autoclave was cooled down to the room temperature.

In a further example, zirconium oxide nanocrystals were prepared from 45millimoles of zirconium isopropoxide isopropanol or zirconium ethoxidemixed with 300 milliliters of benzyl alcohol in an inert atmospherewhich was transferred to an autoclave. The reaction mixture was heatedto 300-350° C. for 1-2 hours at a heating rate of 10° C./min. Thepressure of the reaction reaches 100 to 500 psi. After the reaction wascomplete and the reactor was returned to room temperature, a white milkysolution of as-synthesized zirconium oxide nanocrystals was collected.

An exemplary synthetic method using zirconium n-butoxide as theprecursor is as follows: 21.58 g of 80% (w/w) Zirconium (IV) n-butoxidein 1-butanol solution (containing 17.26 g or 45 mmol Zirconium (IV)n-butoxide) was mixed with 300 ml of benzyl alcohol in a glove box andthen transferred into an autoclave with a glass liner. The setup wassealed under an argon atmosphere to prevent oxygen and moisturecontamination. The autoclave was then heated up to 325° C., kept at thistemperature for one hour and then cooled down to room temperature. Awhite milky solution of as-synthesized zirconium oxide nanocrystals wascollected.

Zirconium n-butoxide is received as a solution in 1-butanol (80% w/w).1-butanol can be removed from the precursor before the synthesis undervacuum and/or heating (30-50° C.), during the synthesis by releasing thepressure of the autoclave when the temperature reaches around 100° C. orafter the reaction is completed. FIG. 4 is the TEM image of thenanocrystals obtained from the reaction without removing 1-butanol. Thenanocrystals are spherical in shape and around 5 nm in diameter.

An exemplary synthetic method using zirconium n-propoxide as theprecursor is as follows: 21.06 g of 70% (w/w) Zirconium (IV) n-propoxidein 1-propanol solution (containing 14.74 g or 45 mmol Zirconium (IV)n-propoxide) was mixed with 300 ml of benzyl alcohol in a glove box andthen transferred into an autoclave. The setup was sealed under Argonatmosphere to prevent oxygen and moisture contamination. The autoclavewas then heated up to 325° C., kept at this temperature for one hour andthen cooled down to room temperature. A white milky solution ofas-synthesized zirconium oxide nanocrystals was collected.

Zirconium n-propoxide is received as a solution in 1-propanol (70% w/w).1-propanol can be removed from the precursor before the synthesis undervacuum and/or upon heating (30-60° C.). It can also be removed duringthe synthesis by releasing the pressure of the autoclave when thetemperature reaches around 100° C. or it can be removed after thesynthesis. The nanocrystals obtained from the reactions where 1-propanolwas removed from the precursor before, during or after the reactionresult in around 5 nm ZrO₂ nanocrystals. These nanocrystals have thesame crystal structure as shown by the respective XRD patterns of thenanocrystals shown in FIG. 5. The nanocrystals obtained by removal of1-propanol before or during the reaction are more spherical andmonodisperse based on a comparison of the TEM images shown in FIG. 6 afor the nanocrystals obtained with removal before the reaction and FIG.6 b without removal of 1-propanol.

To increase the yield of the reaction without affecting the nanocrystalquality the concentration of the precursor, such as zirconiumisopropoxide isopropanol, zirconium etoxide, zirconium n-propoxide orzirconium n-butoxide, can be increased 5-20 times without changing theamount of the solvent used.

ZrO₂ nanocrystals can be synthesized in a variety of solvents andsolvent mixtures. A change in the solvent used in the synthetic methodcan lead to a change in the surface properties of the nanocrystals and,in some cases, can cap the nanocrystals well enough that further surfacemodification in order to obtain dispersions with minimal scattering maybe unnecessary. A list of alternative solvents includes, but is notlimited to: 1-hexanol, oleyl alcohol, oleylamine, trioctylamine, andmethyl triethylene glycol. A list of alternative solvent mixturesincludes, but is not limited to: mixtures of benzyl alcohol with1-hexanol, oleyl alcohol, triethylene glycol monomethyl ether andtrioctylamine.

ZrO₂ may also be synthesized in a different manner in order to preparenanocrystals with a hydrophobic surface chemistry. This may be usefulfor applications which benefit from the use of hydrophobic solvents tocreate dispersions of nanocrystals. An example of the synthetic methodto produce ZrO₂ nanocrystals with hydrophobic surface is as follows: thesolvent for the ZrO₂ nanocrystals synthesis contains a mixture of oleylalcohol and benzyl alcohol with different volume ratios. The volumeratio of oleyl to benzyl alcohol in which the reaction is run may bechosen from the following non-limiting list of ratios: 1:3, 1:1, or pureoleyl alcohol. In a typical reaction, 3 millimole of zirconiumisopropoxide isopropanol is added to a 20 ml mixture containing 10 mlanhydrous benzyl alcohol and 10 ml oleyl alcohol in an inert atmosphere.The mixture is stirred for approximately one hour. The reaction mixtureis then added to an autoclave reactor under an inert atmosphere. Thenthe reactor is heated to 325° C. and maintained at 325° C. for 1 hourwith stirring. After cooling the nanocrystals are precipitated out ofthe solution with ethanol.

The exemplary synthetic methods described herein are carried out in anautoclave at temperatures which may be higher than the boiling point ofsome of the solvents used. This can generate pressures in the 100-900psi range, typically around 250 psi. To eliminate the high pressureswhich may normally be present in the ZrO₂ nanocrystals synthesis, asolvent or a mixture of solvents with higher boiling points may be used.One, non-limiting, example of a higher boiling point solvent is DowthermMX, a mixture of alkylated aromatics, from Dow Chemicals. Dowtherm MXcan be used alone or in combination with other solvents such as benzylalcohol. When used alone for the ZrO₂ nanocrystal synthesis, thepressure in the autoclave reactor is less than 100 psi, and typicallyless than 20 psi.

A typical example of a ZrO₂ nanocrystal synthesis carried out in amixture of benzyl alcohol and Dowtherm MX is as follows: 100 ml ofDowtherm MX, 8.13 millimoles of Zirconium Isopropoxide isopropanol and30 ml of Anhydrous Benzyl Alcohol are mixed in a 250 ml flask for 30 minwith magnetic stirrer at 500 rpm in a glove box. The mixture is thenloaded in to a 600 ml glass-lined Parr autoclave reactor. The reactorwas then sealed in the glove box. The reaction mixture is heated to 325°C. at heating rate of 10° C./min while stirring and kept at thistemperature for 1 hour with stirring. After that it was cooled to roomtemperature and a milky white suspension of ZrO₂ nanocrystals isobtained.

A typical example of the procedure for a ZrO₂ nanocrystal synthesisusing only Dowtherm MX as the solvent follows: 100 ml of Dowtherm MX ismixed with 3.15 g of Zirconium Isopropoxide isopropanol in a 250 mlflask for 30 min at 500 rpm with magnetic stirrer in a glove box. Themixture is then loaded in to a 600 ml glass-lined Parr Reactor. Thereactor was then sealed while in the glove box, before being transferredout for the reaction. The reaction mixture is heated to 325° C. atheating rate of 10° C./Min min with stirring and kept at thistemperature for 1 hour with stirring. After that it was cooled to roomtemperature and a milky white suspension of ZrO₂ nanocrystals isobtained.

Alternatively, precursors other than zirconium (IV) isopropoxideisopropanol may be used to synthesize ZrO₂ nanocrystals in solvents witha higher boiling point than the reaction temperature, or a mixture ofthese solvents with benzyl alcohol. These alternative precursors mayinclude but are not limited to zirconium (IV) ethoxide, zirconium (IV)n-propoxide, and zirconium (IV) n-butoxide.

Synthesis of 1-5 nm ZrO₂ Nanocrystals

ZrO₂ nanocrystals can be synthesized with average diameters from 1 to 5nm, preferably 1 to 3 nm, by controlling the amount of water in thereaction mixture during the solvothermal synthesis. These smaller sizednanocrystals (1-5 nm) may be desirable for increased specific surfacearea with respect to larger (6-10 nm) nanocrystals or for use inapplications where the smaller physical size may be beneficial. Atypical example of the experimental protocol for the synthesis of thesenanocrystals is as follows: In a vial, 30 ml of benzyl alcohol and 0.08ml of water (4.44 mmol) were stirred for 1 hour and transferred into theglovebox. In the glovebox, 4.49 millimoles of zirconium (IV)isopropoxide isopropanol (Zr(OPr^(i))₄(HOPr^(i)))—, (˜1:1 water toprecursor ratio) was stirred with the benzyl alcohol solution for 4hours. The precursor was completely dissolved into the solvent and aclear solution was obtained. The reaction mixture was then transferredto an autoclave and sealed within the vessel. The reaction mixture wasthen heated at 325° C. for 1 hour (15 minutes ramp up to 250° C., 3minutes ramp up to 265° C., 3 minutes ramp up to 280° C., 3 minutes rampup to 295° C., 3 minutes ramp up to 310° C., 3 minutes ramp up to 325°C.) while stirring. After cooling to room temperature, a white slurryand a faint yellow solution were obtained. The XRD pattern of the solidmatches that of ZrO₂ and the TEM images of the nanocrystals shows thatthe particle size is around 3 nm. FIG. 7 a, b, c, and d show the TEMimages of the nanocrystals obtained from 1:1, 1:2, 1:3 and 1:4 molarratio of precursor to water in the reaction mixture, respectively. FIG.7 shows that as the ratio of water to precursor increased the particlesize gets even smaller with 1:4 water to precursor ratio resulting inthe smallest average particle size (˜2 nm) among the exemplary ratios of1:1, 1:2, 1:3 and 1:4.

Alternatively ZrO₂ nanocrystals may be synthesized with averagediameters from 1 to 5 nm, preferably 1 to 3 nm, using precursors otherthan zirconium (IV) isopropoxide isopropanol. These alternativeprecursors may include zirconium (IV) ethoxide, zirconium (IV)n-propoxide, and zirconium (IV) n-butoxide.

The heating temperature and time of the exemplary synthetic routesdescribed herein for the synthesis of ZrO₂ nanocrystals can be adjustedsuch that the reaction temperature can be varied from 250-350° C. whilethe reaction time can be varied from 20 min-24 hours. Reactions carriedout at the lower end of the temperature range may require longer heatingtimes and the reactions carried out at the higher end of thistemperature range may require shorter times for a complete synthesis.

Synthesis of Titanium-Zirconium Oxide (TiO₂—ZrO₂) Nanocrystals

Metal-oxide nanocrystals containing both zirconium and titanium atomscan be synthesized by a modification of the synthetic route for ZrO₂nanocrystals. These TiO₂—ZrO₂ metal oxide nanocrystals may be used in avariety of applications which call for the mixture of chemicalproperties, physical properties, or optical properties (or somecombination therein) of ZrO₂ and TiO₂. One set of non-limiting examplesof this TiO₂—ZrO₂ synthesis involves replacing the zirconium precursorwith a mixture containing both a titanium precursor and a zirconiumprecursor in benzyl alcohol. Nanocrystals with different Ti/Zr atomicratios can be made by adjusting the titanium and zirconium precursorconcentrations with respect to each other while holding the total metalprecursor concentration constant. TiO₂—ZrO₂ nanocrystals can besynthesized in this manner with the Ti:Zr ratio taking a value from thefollowing non-limiting list: 1:3, 1:2, and 1:1.

A typical procedure for the synthesis of TiO₂—ZrO₂ nanocrystals with 1:1Ti:Zr ratio is as follows: 15 mmol of zirconium isopropoxide isopropanoland 15 mmol of titanium isopropoxide were dissolved in 30 ml anhydrousbenzyl alcohol under an inert atmosphere. The reaction mixture was thenadded to an autoclave reactor under an inert atmosphere. The reactor washeated to 300° C. and maintained at 300° C. for 1 hour with stirring.The resulting nanocrystals were precipitated out of solution withethanol. The TiO₂—ZrO₂ nanocrystals have a size of around 5 nm based onTEM images. The elemental analysis results confirmed that the Ti/Zratomic ratio in the sample was generally consistent with the atomicratio of the two precursors.

A typical procedure for the synthesis of TiO₂—ZrO₂ nanocrystals with aTi:Zr ratio of 1:2 involves the following: 20 mmol of zirconiumisopropoxide isopropanol and 10 mmol of titanium isopropoxide weredissolved in 30 ml anhydrous benzyl alcohol under an inert atmosphere.The reaction mixture was then added to an autoclave reactor under aninert atmosphere. The reactor was heated to 300° C. and maintained at300° C. for 1 hour with stirring. The resulting nanocrystals wereprecipitated out of solution with ethanol.

Alternatively the synthesis of TiO₂—ZrO₂ nanocrystals with variousvalues of x may be synthesized using a mixture of titanium and zirconiumwhich is not a mixture of zirconium isopropoxide isopropanol andtitanium isopropoxide. The mixture of zirconium and titanium precursorsmay include a zirconium precursor from a non-limiting list including:zirconium ethoxide, zirconium n-propoxide, and zirconium n-butoxide, anda titanium precursor including titanium ethoxide, titanium n-propoxide,and titanium n-butoxide.

Synthesis of Hafnium-Zirconium Oxide (HfO₂—ZrO₂) Nanocrystals

Metal-oxide nanocrystals containing both zirconium and hafnium atoms ina single nanocrystal can be synthesized. HfO₂—ZrO₂ oxide nanocrystalswith an 1:1 atomic ratio of hafnium to zirconium can be produced in aninert atmosphere by mixing 2 millimoles of hafnium isopropoxideisopropanol and 2 millimoles of zirconium chloride with 10 grams oftrioctylphosphine oxide. The reaction mixture is then heated to 100° C.,at a heating rate of 10° C./min, with vigorous stirring under an inertatmosphere. After 1 hour stirring at 100° C., trioctylphosphine oxide ismelted and the hafnium and zirconium precursors are dissolved in meltedtrioctylphosphine oxide. The solution is then rapidly heated to 350° C.,at a heating rate of 10° C./min, and kept at 350° C. for two hours. Awhite powder appeared and the solution became milky. After two hours,the reaction mixture is allowed to cool. When the reaction mixturereached 70° C., acetone is added, causing the nanocrystals toprecipitate. The resulting hafnium-zirconium oxide nanomaterial isrod-like in shape (i.e., “nanorods”).

In a further example, hafnium-zirconium oxide nanocrystals, may beprepared with a range of values for the hafnium to zirconium atomicratio. For example, nanocrystals with a Hf:Zr ratio of 1:4 can beprepared with the following: 0.8 mmol of hafnium isopropoxideisopropanol, 1.2 mmol of zirconium isopropoxide isopropanol, 2 mmol ofzirconium chloride, and 10 grams of trioctylphosphine oxide are mixedtogether in an inert atmosphere. The feeding sequence is arbitrary. Thereaction mixture is heated to 100° C., at a heating rate of 10° C./min,with vigorous stirring under an inert atmosphere. The solution is thenrapidly heated to 350° C., at a heating rate 10° C./min, and kept at350° C. for two hours. A white powder forms and the solution becomesmilky. After two hours, the reaction mixture is allowed to cool. Whenthe reaction mixture reached 70° C., acetone is added, causing theHfO₂—ZrO₂ nanocrystals to precipitate. The precipitate is collected bycentrifugation and the supernatant is decanted and discarded. Theredispersion-precipitation procedure is repeated 4 times. The shape ofhafnium zirconium oxide nanomaterials range from spheres to rod-like(i.e., “nanorods”).

Synthesis of Hafnium Oxide (HfO₂) Nanocrystals

Hafnium oxide nanocrystals having a size in the range of 1-10 nm aresynthesized in an inert atmosphere using a solvothermal syntheticmethod. An example of the synthetic method is as follows: a sample ofhafnium alkoxide precursor, such as, but not limited to, hafniumisopropoxide isopropanol, or hafnium ethoxide, was mixed with an organicalcohol, such as, but not limited to, benzyl alcohol or2-methyl-1-propanol, and sealed within an autoclave. The reactionmixture was heated to 250-350° C. Once the reaction mixture reached theset temperature, the temperature was maintained for a set time which canrange from 20 minutes to 24 hours. As-synthesized hafnium oxidenanocrystals were collected as a white milky suspension. FIG. 8 shows aTEM image of the as synthesized HfO₂ nanocrystals which have a riceshape and are less than 10 nm in size.

A method of producing 6 g of hafnium oxide nanocrystals of the presentdisclosure includes mixing, in an inert atmosphere, a sample of 30millimoles of hafnium ethoxide or hafnium isopropoxide isopropanol with300 milliliters of benzyl alcohol which is then transferred to anautoclave. The reaction mixture is heated at 300-350° C. for 1-2 hours,with a heating ramp rate of 10° C./min. During reaction the pressure inthe autoclave is less than 500 psi (˜35 atmospheres). After the reactiontime has elapsed and the reactor is returned to room temperature, awhite milky solution of as-synthesized hafnium oxide nanocrystals iscollected.

Alternatively, precursors other than hafnium (IV) isopropoxideisopropanol or hafnium ethoxide, may be used to synthesize HfO₂nanocrystals in solvents with a higher boiling point than the reactiontemperature, or a mixture of these solvents with benzyl alcohol. Thesealternative precursors may include but are not limited to hafnium (IV)n-propoxide, and hafnium (IV) n-butoxide.

Synthesis of 1-5 nm HfO₂ Nanocrystals

HfO₂ nanocrystals can be synthesized with diameters of 1-5 nm,preferably 1-3 nm, by controlling the amount of water in the reactionmixture during the solvothermal synthesis. These smaller sizednanocrystals may be desirable for their increased specific surface areawith respect to larger nanocrystals or for use in applications where thesmaller physical size may be beneficial. A typical example of theexperimental protocol for the addition of water in order to producehafnium oxide nanocrystals in the 1-5 nm size range follows: 30 ml ofbenzyl alcohol and 0.1 ml of water are stirred for 3 hours in a vialwhich is then transferred into the drybox. In the drybox, 4.45 millimoleof Hf(OPr^(i))₄(HOPr^(i)) (2.113 g) is stirred in the water/benzylalcohol solution overnight with 1:1 water to hafnium isopropoxide molarratio. The precursor completely dissolves into the solvent mixture. Thereaction mixture is transferred to an autoclave and sealed within thevessel. The reaction mixture is then heated to 325° C., using a heatingmantle, for 1 hour with stirring. After cooling to room temperature, awhite slurry with a faint yellow solution are obtained. FIG. 9 shows theTEM images of the nanocrystal which are 2-5 nm in size.

Alternatively, HfO₂ nanocrystals may be synthesized with diameters of1-5 nm, preferably 1-3 nm, starting from precursors other than hafniumisopropoxide isopropanol. These alternative precursors may include butare not limited to hafnium ethoxide, hafnium n-propoxide, and hafniumn-butoxide.

Hafnium alkoxide to water ratio can be in the range from 1:1 to 1:4.

The heating temperature and time of the exemplary synthetic routesdescribed herein for the synthesis of HfO₂ nanocrystals can be adjustedsuch that the reaction temperature can be varied from 250-350° C. whilethe reaction time can be varied from 20 min-24 hours. Reactions carriedout at the lower end of the temperature range may require longer heatingtimes and the reactions carried out at the higher end of thistemperature range may require shorter times.

Synthesis of Zinc Oxide (ZnO) Nanocrystals

Organosilane capped zinc oxide nanocrystals were produced as follows.2.7 grams of zinc acetate dihydrate were dissolved in 140 ml of ethanoland heated to 80° C. with stirring. Once the zinc acetate was completelydissolved and the solution turned clear, the reaction mixture was cooledin an ice-water bath. In a separate flask, a 0.72 gram sample of lithiumhydroxide monohydrate was mixed with 60 milliliters of ethanol andsonicated for 30 minutes. This lithium hydroxide/ethanol solution wasadded drop-wise, at a rate of 3 drops per second, to the zinc acetatedihydrate/ethanol solution in the ice-water bath. Once the entirelithium hydroxide/ethanol solution was added, the reaction mixture waswarmed to room temperature and stirred for 1 hour. A 0.25 gram sample ofmethoxy(triethyleneoxypropyl)trimethoxysilane was mixed with 5milliliters of ethanol and then injected into the reaction mixture. Theentire reaction mixture was stirred for 12 hours at room temperature,forming as-synthesized organosilane-capped zinc oxide nanocrystals.These nanocrystals have a spherical shape with diameters in the 3-6 nmrange.

In a further example, larger-sized (equal to or greater than 5 nm andless than 10 nm) organosilane-capped zinc oxide nanocrystals wereproduced as follows: 2.7 grams of zinc acetate dihydrate was dissolvedin 140 milliliters of ethanol and heated to 80° C. with stirring. Oncethe zinc acetate was completely dissolved and the solution turned clear,the reaction mixture was cooled in an ice-water bath. In a separateflask, a 0.72 gram sample of lithium hydroxide monohydrate was mixedwith 60 milliliters of ethanol and sonicated for 30 minutes. Thissolution was added drop-wise, at a rate of 3 drops per second, to thezinc acetate dihydrate/ethanol solution in the ice-water bath. Once theentire lithium hydroxide/ethanol solution was added, the reactionmixture was placed into a 60° C. hot water bath and stirred for 1 hour.A 0.25 gram sample of methoxytri(ethyleneoxy)propyltrimethoxysilane wasmixed with 5 milliliters of ethanol and then injected into the reactionmixture. The entire reaction mixture was stirred for 12 hours at 60° C.,forming the as synthesized organosilane capped zinc oxide nanocrystalswith diameters equal to or greater than 5 nm and less than 10 nm.

A method of producing organosilane-capped zinc oxide nanocrystals isprovided. A 21.28 gram sample of zinc acetate dihydrate is dissolved in1080 ml of ethanol and heated to ˜80° C. with stirring. Once the zincacetate is completely dissolved and the solution turned clear, thereaction mixture is cooled in an ice-water bath. In a separate flask, a5.76 gram sample of lithium hydroxide monohydrate is mixed with 480 mlof ethanol and sonicated for 30 minutes. This solution is addeddrop-wise to the zinc acetate dihydrate/ethanol solution in theice-water bath. Once the entire lithium hydroxide/ethanol solution isadded, the reaction mixture is warmed to room temperature and stirredfor 0.5 hours. A 2.0 gram sample ofmethoxytri(ethyleneoxy)propyltrimethoxysilane is mixed with 15milliliters of ethanol and then injected into the reaction mixture. Theentire reaction mixture is stirred for 16 hours at room temperature,forming as-synthesized organosilane capped zinc oxide nanocrystals.These nanocrystals are spherical with 3-6 nm in diameter.

Alternatively, 4 times more concentrated methoxytri(ethyleneoxy)propyltrimethoxysilane with respect to ethanol was added to the reactionmixture during the synthesis of 3-6 and 5-10 nm ZnO nanocrystals toprovided increased capping and dispersibility of the nanocrystals inpolar solvents.

ZnO nanocrystals can be synthesized by another liquid synthetic method.A typical synthesis is as follows: 50 mmol zinc acetate dihydrate wasadded to 500 ml of absolute ethanol in a flask. The zinc acetate wasdissolved completely by heating the flask in a water bath at 80° C.Separately, 200 mmol lithium hydroxide monohydrate was dissolved in 125ml methanol (or ethanol) at room temperature by vigorous stirring. TheLiOH solution was then poured into the refluxing Zn(Ac)₂ solution.Following the addition, the heat was removed and the reaction mixturewas cooled in air for 20 minutes. A transparent solution resulted. Thissolution was then re-heated to 80° C. for 30 minutes, until a whiteprecipitate formed. The precipitate is separated from the solution bycentrifuging at 4500 rpm at 4° C. for 20 minutes, and washed with THF. ATEM image of the product is shown in FIG. 10.

Alternatively, in the above reactions used to produce ZnO nanocrystals,the molar ratio of the lithium hydroxide to zinc salt can be varied inthe range from 1:1.4 to 1:4.

Alternatively, in the above reactions used to produce ZnO nanocrystals,KOH or NaOH can be used as a substitute for lithium hydroxide.

Synthesis of Yttrium Oxide (Y₂O₃) Nanocrystals

Yttrium oxide nanocrystals were produced from 1 gram of yttrium oleateand 5.96 grams of dodecylamine which were mixed together and purged withan inert gas for 10 minutes. The reaction mixture was then heated to 70°C. in 20 minutes, maintained at 70° C. for 20 minutes, then furtherheated to 259° C. in 20 minutes and maintained at 259° C. for 2 hours,with stirring under an inert atmosphere. The reaction mixture was thenallowed to cool. At 70° C., 20 ml of ethanol were added to the reactionmixture to precipitate the yttrium oxide nanocrystals.

In another example, yttrium oxide nanodisks (disk-shaped nanocrystals)with a diameter of 20 nanometers were produced from a mixture of 1 gramof yttrium oleate and 5 ml of oleylamine which were mixed and purgedwith an inert gas such as Argon for 10 minutes. The reaction mixture wasthen heated to 70° C. in 20 minutes, maintained at 70° C. for 20minutes, heated to 250° C. in 20 minutes, and finally maintained at 250°C. for 2 hours while stirring under an inert gas atmosphere. Thereaction mixture was then allowed to cool. At 70° C., 20 milliliters ofethanol was added to the reaction mixture to precipitate the yttriumoxide nanodisks.

In another example, yttrium oxide nanodisks with a diameter of 10nanometers were produced from 2 grams of yttrium oleate and 25 ml ofoleylamine which were mixed and purged with argon for 10 minutes. Thereaction mixture was then heated to 70° C. in 20 minutes, maintained at70° C. for 20 minutes, heated to 280° C. in 20 minutes, and finallymaintained at 280° C. for 2 hours while stirring under argon protection.The reaction mixture was then allowed to cool. At 70° C., 20 millilitersof ethanol was added to the reaction mixture to precipitate the yttriumoxide nanodisks.

In a further example, yttrium oxide nanodisks with a diameter of 10nanometers were produced from 2 grams of yttrium oleate and 25 ml ofoleylamine which were mixed and purged with argon for 10 minutes. Thereaction mixture was then heated to 70° C. in 20 minutes, maintained at70° C. for 20 minutes, heated to 230° C. in 20 minutes, and finallymaintained at 230° C. for 2 hours while stirring under argon protection.The reaction mixture was then allowed to cool. At 70° C., 20 millilitersof ethanol was added to the reaction mixture to precipitate out theyttrium oxide nanodisks.

In a further example, yttrium oxide nanocrystals were produced from 2.15grams of yttrium oleate and 23 grams of dodecylamine which were mixedtogether and purged with an inert gas for 10 minutes. The reactionmixture was then heated to 70° C. in 20 minutes, maintained at 70° C.for 20 minutes, then heated to 259° C. in 20 minutes and maintained at259° C. for 2 hours, with stirring under an inert atmosphere. Thereaction mixture was then allowed to cool. At 70° C., 20 milliliters ofethanol was added to the reaction mixture to precipitate the yttriumoxide nanocrystals. The product has a flake-like shape, where the flakeshave a thickness of 2 nm.

Example 2 Removing Ligands from Surface of Nanocrystals

Hydrochloric acid treatment of the as-synthesized HfO₂ and ZrO₂nanocrystal surface may be necessary to remove the organic moieties orcapping agents which are on the surface of the nanocrystals before anyfurther modification is possible. An exemplary method includessuspending as-synthesized or purified nanocrystals in water by stirringand adjusting the suspension to a pH of 1 using a 1 M hydrochloric acidsolution. The solution changes from a milky white suspension to atransparent solution upon the addition of hydrochloric acid. Thesolution may be stirred overnight at room temperature to allow thereaction to progress further. When the solution is added totetrahydrofuran, a white solid precipitates. After centrifugation, theprecipitate can be collected. The process of re-suspending the particlesin tetrahydrofuran and then centrifuging the mixture and collecting theprecipitate may be repeated until the pH of the supernatant is in the5-7 range.

Example 3 Cap Exchange of Nanocrystals

Cap exchange of ZrO₂, HfO₂ and TiO₂—ZrO₂ Nanocrystals

After the synthesis of the ZrO₂, HfO₂ and TiO₂—ZrO₂ nanocrystals, theas-synthesized nanocrystals are transferred into a round bottom flask toperform the cap exchange. The as-produced nanocrystals may be capped bythe solvent or reaction by-products that are present during synthesis.It may be desirable to exchange the capping molecules of thenanocrystals for a variety of reasons, including, but not limited to:increased dispersibility in solvent or some other matrix, havingdifferent optical properties, or having different chemistry at thesurface of the nanocrystals. The cap exchange process may involvedispersing or suspending the as-synthesized nanocrystals in a solvent orreaction mixture along with a certain amount of capping agents. Thisreaction may be carried out at an elevated temperature and for a certainamount of time in order to promote cap exchange. A non-limiting list ofchoices for capping agents to perform the cap exchange on theas-synthesized ZrO₂, HfO₂ and TiO₂—ZrO₂ nanocrystals includes:methoxytri(ethelyneoxy)propyltrimethoxy silane,2-[2-(2-methoxyethoxy)ethoxy]acetic acid, 3-(methacryloyloxy)propyltrimethoxysilane and other silanes, carboxylic acids and alcohols. Thecap exchange may be carried out in benzyl alcohol or other solvent ormixtures of solvents.

Cap exchange of the as-synthesized ZrO₂, HfO₂ and TiO₂—ZrO₂ nanocrystalsmay be carried out using methoxytri(ethelyneoxy)propyltrimethoxy silaneas the capping agent. The methoxytri(ethelyneoxy)propyltrimethoxy silanemay be injected into a reaction vessel (typically a round bottom flask)containing the as-synthesized nanocrystals reaction mixture. The weightratio of methoxytri(ethelyneoxy)propyltrimethoxy silane to the assynthesized nanocrystals may range from 1:5 to 3:2. Then the mixture isheated to 80-150° C. for an interval that may be as short as 10 minutesor as along as 3 hours. A typical procedure for amethoxy(triethelyneoxy)propyltrimethoxy silane cap exchange onas-synthesized nanocrystals involves the following: 1 g ofmethoxy(triethelyneoxy)propyltrimethoxysilane capping agent was added toa round bottom flask which holds the reaction mixture containing 5 g ofas-synthesized ZrO₂, HfO₂ or TiO₂—ZrO₂ nanocrystals. During the additionof the capping agent the mixture was stirred continuously. Thesuspension was heated up to 80-150° C. and kept at that temperaturewhile continuing to stir for 10 min-1 hour. Afterwards, the reaction wasallowed to cool to room temperature.

Alternatively, the cap exchange of the ZrO₂, HfO₂ and TiO₂—ZrO₂nanocrystals with methoxy(triethelyneoxy)propyltrimethoxy silane as thecapping agent may be carried out on suspensions of nanocrystals otherthan the as-synthesized reaction mixture. Similar reactions may becarried out on suspensions of nanocrystals including, but not limitedto, suspensions containing: nanocrystals which have previously undergonecap-exchange, as-synthesized nanocrystals which have previouslyundergone purification, nanocrystals which have had the capping agentsremoved by acid treatment, and nanocrystals which have been transferredto different solvents. Alternative solvents for cap exchange may bechosen from a list including, but not limited to: benzyl alcohol,propylene glycol monomethyl ether (PGME), propylene glycol methyl etheracetate (PGMEA), ethyl lactate (EL), and 2-propoxy-propanol (PnP),acetone, tetrahydrofuran, phenol, oleyl alcohol, toluene, butanol,propanol, isopropanol, ethanol, water and mixtures thereof.

Cap exchange of the as-synthesized ZrO₂, HfO₂ and TiO₂—ZrO₂ nanocrystalsmay be carried out using 2-[2-(2-methoxyethoxy)ethoxy]acetic acid as thecapping agent. The 2-[2-(2-methoxyethoxy)ethoxy]acetic acid may beinjected into a reaction vessel (typically a round bottom flask)containing the as-synthesized nanocrystals reaction mixture. The amountof 2-[2-(2-methoxyethoxy)ethoxy]acetic acid may be as little as 0.4 g ormay be as much as 1.5 g per gram of as-synthesized ZrO₂, HfO₂ orTiO₂—ZrO₂ nanocrystals. Then the mixture may either be kept at atemperature as low as 20° C. or heated as high as 50° C. for an intervalthat may be as short as 30 minutes or as along as 3 hours. A typicalprocedure for a 2-[2-(2-methoxyethoxy)ethoxy]acetic acid cap exchangereaction performed on as-synthesized nanocrystals involves thefollowing: 2 g of 2-[2-(2-methoxyethoxy)ethoxy]acetic acid is added to around bottom flask which holds the reaction mixture containing 5 g ofas-synthesized nanocrystals. During the addition the mixture is stirredcontinuously. The suspension is kept at room temperature whilecontinuing to stir for 1 hour.

Alternatively, the cap exchange of the ZrO₂, HfO₂ and TiO₂—ZrO₂nanocrystals with 2-[2-(2-methoxyethoxy)ethoxy]acetic acid as thecapping agent may be carried out on suspensions of nanocrystals otherthan the as-synthesized reaction mixture. Similar reactions may becarried out on suspensions of ZrO₂, HfO₂ or TiO₂—ZrO₂ nanocrystalsincluding, but not limited to, suspensions containing: nanocrystalswhich have previously undergone cap-exchange, as-synthesizednanocrystals which have previously undergone purification, nanocrystalswhich have had the capping agents removed by acid treatment, andnanocrystals which have been transferred to different solvents.Alternative solvents for cap-exchange reactions may be chosen from alist including, but not limited to: benzyl alcohol, propylene glycolmonomethyl ether (PGME), propylene glycol methyl ether acetate (PGMEA),ethyl lactate (EL), and 2-propoxy-propanol (PnP), acetone,tetrahydrofuran, phenol, oleyl alcohol, toluene, butanol, propanol,isopropanol, ethanol, water, cyclic ketones and mixtures thereof.

Cap exchange of the as-synthesized ZrO₂, HfO₂ and TiO₂—ZrO₂ nanocrystalsmay be carried out using 3-(methacryloyloxy)propyl trimethoxysilane asthe capping agent. The 3-(methacryloyloxy)propyl trimethoxysilane may beinjected into a reaction vessel (typically a round bottom flask)containing the as-synthesized nanocrystals reaction mixture. The amountof 3-(methacryloyloxy)propyl trimethoxysilane may be as little as 0.8 gor may be as much as 1.5 g per gram of as-synthesized nanocrystals. Thenthe mixture is heated to 120° C. for an interval that may be as short as30 minutes or as along as 1 hour. A typical procedure for a3-(methacryloyloxy)propyl trimethoxysilane cap exchange performed onas-synthesized nanocrystals involves the following: 4 g of3-(methacryloyloxy)propyl trimethoxysilane is added to a round bottomflask which holds the reaction mixture containing 5 g of as-synthesizedZrO₂, HfO₂ or TiO₂—ZrO₂ nanocrystals. During the addition of the cappingagent the mixture is stirred continuously. The suspension is heated upto 120° C. and kept at that temperature while continuing to stir for 1hour. Afterwards, the reaction is allowed to cool to room temperature.

Alternatively, the cap exchange of the ZrO₂, HfO₂ and TiO₂—ZrO₂nanocrystals with 3-(methacryloyloxy)propyl trimethoxysilane as thecapping agent may be carried out on suspensions of nanocrystals otherthan the as-synthesized reaction mixture. Similar reactions may becarried out on suspensions of nanocrystals including, but not limitedto, suspensions containing: nanocrystals which have previously undergonecap-exchange, as-synthesized nanocrystals which have previouslyundergone purification, nanocrystals which have had the capping agentsremoved by acid treatment, and nanocrystals which have been transferredto different solvents. Alternative solvents for dispersion of thenanocrystals during the cap exchange reaction may be chosen from a listincluding, but not limited to: benzyl alcohol, propylene glycolmonomethyl ether (PGME), propylene glycol methyl ether acetate (PGMEA),ethyl lactate (EL), and 2-propoxy-propanol (PnP), acetone,tetrahydrofuran, phenol, oleyl alcohol, toluene, butanol, propanol,isopropanol, ethanol, water, cyclic ketones and mixtures thereof.

Cap exchange of the as-synthesized ZrO₂, HfO₂ and TiO₂—ZrO₂ nanocrystalsmay be carried out using 3-(methacryloyloxy)propyl trimethoxysilane andmethoxy(triethelyneoxy)propyltrimethoxy silane as capping agents. Anexemplary cap exchange reaction of ZrO₂ nanocrystals is as follows: 500mg as synthesized ZrO₂ was mixed with 25 mg 3-(methacryloyloxy)propyltrimethoxysilane in 5 ml PGMEA at 100° C. for 1 hour. 150 mg asmethoxy(triethelyneoxy)propyltrimethoxy silane is then added to thesuspension and the mixture was stirred at 100° C. for another hour. Theproduct mixture was washed with heptanes and white precipitate iscollected.

The as-produced nanocrystals of ZrO₂, HfO₂ and TiO₂—ZrO₂ may also becapped in order to facilitate dispersion in hydrophobic solvents andmatrices. The cap exchange process may involve dispersing or suspendingthe as-synthesized nanocrystals, along with a certain amount of cappingagent or capping agents, in a relatively hydrophobic solvent, chosenfrom a list of solvents including but not limited to: naptha, toluene,heptane, pentane, decane, chloroform. This cap exchange reaction may becarried out at room temperature or an elevated temperature and for anamount of time ranging from a few minutes to days in order to promotecap exchange. A list of choices for capping agents that may make thesurface of the as-synthesized ZrO₂, HfO₂ and TiO₂—ZrO₂ nanocrystals morecompatible with hydrophobic solvents and media includes, but is notlimited to: stearic acid, oleic acid, and octadecyltrimethoxysilane. Ina typical reaction: 2 g oleic acid is added to a suspension containing 2g of as-synthesized nanocrystals in 20 ml of toluene. During and afterthe addition of the capping agent the mixture is continuously stirred.The reaction mixture is allowed to react for between several minutes andseveral hours before purification is then carried out.

Cap exchange of the as-synthesized ZrO₂, HfO₂ and TiO₂—ZrO₂ nanocrystalsmay be carried out using methoxypoly(ethelyneoxy)propyltrimethoxy silaneas capping agent. Alternatively, the cap exchange of the ZrO₂, HfO₂ andTiO₂—ZrO₂ nanocrystals with methoxypoly(ethelyneoxy)propyltrimethoxysilane as the capping agent may be carried out on suspensions ofnanocrystals other than the as-synthesized reaction mixture. Similarreactions may be carried out on suspensions of nanocrystals including,but not limited to, suspensions containing: nanocrystals which havepreviously undergone cap-exchange, as-synthesized nanocrystals whichhave previously undergone purification, nanocrystals which have had thecapping agents removed by acid treatment, and nanocrystals which havebeen transferred to different solvents. Alternative solvents fordispersion of the nanocrystals during the cap exchange reaction may bechosen from a list including, but not limited to: benzyl alcohol,propylene glycol monomethyl ether (PGME), propylene glycol methyl etheracetate (PGMEA), ethyl lactate (EL), and 2-propoxy-propanol (PnP),acetone, tetrahydrofuran, phenol, oleyl alcohol, toluene, butanol,propanol, isopropanol, ethanol, water, cyclic ketones and mixturesthereof.

Cap-Exchange of Yttrium Oxide Nanocrystals

Organosilane capped yttrium oxide nanocrystals can be produced via a capexchange process involving as synthesized yttrium oxide nanocrystals andmethoxy(triethyleneoxy)propyltrimethoxysilane. As-produced yttrium oxidenanocrystals and methoxytri(ethyleneoxy)propyltrimethoxysilane weremixed together in tetrahydrofuran. The mixture was then heated to 200°C. for 2-4 hours inside an autoclave. After the reaction time expiredthe mixture was allowed to cool to room temperature.

Alternatively the cap exchange process may be carried of otherorganosilanes, organocarboxylic acids and organoalcohols. Similarreactions may be carried out on suspensions of nanocrystals including,but not limited to, suspensions containing: nanocrystals which havepreviously undergone cap-exchange, as-synthesized nanocrystals whichhave previously undergone purification, nanocrystals which have had thecapping agents removed by acid treatment, and nanocrystals which havebeen transferred to different solvents. Alternative solvents fordispersion of the nanocrystals during the cap exchange reaction may bechosen from a list including, but not limited to: benzyl alcohol,propylene glycol monomethyl ether (PGME), propylene glycol methyl etheracetate (PGMEA), ethyl lactate (EL), and 2-propoxy-propanol (PnP),acetone, phenol, oleyl alcohol, toluene, butanol, propanol, isopropanol,ethanol, water, cyclic ketones and mixtures thereof.

Cap-Exchange of ZnO Nanocrystals

When the ZnO nanocrystals are synthesized without the addition of acapping agent during the synthesis, they can be capped after thesynthesis is complete with 3-(methacryloyloxy)propyl trimethoxysilane,methoxytri(ethelyneoxy)propyltrimethoxy silane,2,2,2-methyoxyethyoxyethyoxy-acetic acid or a combination of thesematerials. The capping with 2,2,2-methyoxyethyoxyethyoxy-acetic acid canbe carried out at room temperature or with the aid of sonication orheating of the suspension to 80° C. or with a combination of bothheating and sonication. A typical method is as follows: After thesynthesis 4 g of as-synthesized precipitate is re-dispersed in PGMEA ina round bottom flask. To this suspension 2 g of2,2,2-methyoxyethyoxyethyoxy-acetic acid is added while stirring. Thesuspension is then exposed to brief (<1 minute) sonication to aid in thecapping reaction. The capped nanocrystals are then precipitated out withTHF and heptane, with the 1:1:3 volume ratio of nanocrystal:THF:heptane.The precipitates are collected by centrifugation at 6500 rpm.

Example 4 Purifying Nanocrystals

As-Synthesized ZrO₂, HfO₂ and TiO₂—ZrO₂ Nanocrystals

The as synthesized white milky nanocrystal suspension collected afterthe autoclave synthesis of the ZrO₂, HfO₂ and TiO₂—ZrO₂ nanocrystals canbe purified. An exemplary method includes mixing the suspensions ofnanocrystals with ethanol and centrifuging (8000 rpm for 30 minutes) toseparate the nanocrystals. After decanting and discarding thesupernatant, a white precipitate is collected. The wet nanocrystals aresuspended in additional ethanol by sonication, stirring or shaking andsuspension is centrifuged again. These resuspension steps, which consistof ethanol addition, centrifugation and collection of the resultantpowder are repeated as many as 4 more times to obtain purifiednanocrystals.

ZrO₂ Nanocrystals with Hydrophobic Surface

To purify the nanocrystals they are dispersed in hexane, and thenprecipitated out using ethanol as the antisolvent. The resultant mixtureis then centrifuged and the nanocrystals are collected. Thispurification process is repeated three times to get nanocrystals thatare easily dispersible into hydrophobic solvents such as naphtha andheptane.

ZrO₂, HfO₂ and TiO₂—ZrO₂ Nanocrystals

After synthesis, capping and or cap exchange, ZrO₂, HfO₂ and TiO₂—ZrO₂nanocrystals may be purified or further purified. One exemplarypurification of the nanocrystals after being synthesized in benzylalcohol or mixture of benzyl alcohol with other solvents may include:addition of THF to the reactions mixture in a 2:1 volume ratio of THF tothe reaction mixture followed by addition of heptane in a 7-9 to 1volume ratio of heptane to the reaction mixture. The reaction ofnanocrystal suspension to THF to heptane may be adjusted based on thenanocrystal concentration in the suspension. This causes theprecipitation of the nanocrystals which are then centrifuged. Aftercentrifugation and the decanting of the supernatant, additional amountsof THF or PGMEA is added to disperse the nanocrystals followed byaddition of heptane. Heptane to THF or PGMEA ratio may be 2:1 or 3:1.Cycles of sonication, centrifugation and decantation is repeated 2-5times to purify the nanocrystals.

ZnO Nanocrystals

As-synthesized, capped and/or cap-exchanged zinc oxide nanocrystals maybe purified or further purified to obtain an optically clear suspensionin a polar solvent. This process removes at least part of the by-productof the synthesis or cap exchange reactions. An exemplary method ofpurifying the ZnO nanocrystals is as follows: A suspension of 200 mlzinc oxide nanocrystals in ethanol (˜1 g ZnO) is mixed with 400-500milliliters of heptane to form a white precipitate which is collected bycentrifugation, followed by decanting and discarding the supernatant. Asample of 20-60 milliliters of ethanol is then used to redisperse thewhite precipitate into solution with 5 minutes of ultrasonication, and asample of 40-50 milliliters of heptane was used again to precipitate theproduct. After collecting the white precipitate by centrifugation, thedecanting and discarding of the supernatant was repeated for a secondtime. The ethanol redispersion/heptane precipitation procedure wasrepeated twice more to obtain a purified nanocrystals.

In a further example, capped zinc oxide nanocrystals were purified toobtain re-dispersable dry powders. A suspension of 200 ml organosilanecapped zinc oxide nanocrystals in ethanol (˜1 g ZnO) was mixed with400-500 ml of heptane to form a white precipitate. This whiteprecipitate was collected by centrifugation, followed by the decantingand discarding of the supernatant. A sample of 20 ml of ethanol was thenused to redisperse the white solid, with the aid of 5 minutes ofultasonication. 40-50 ml of heptane was used to once again precipitatethe product. After collecting the white precipitate by centrifugation,and decanting and discarding the supernatant for a second time, theethanol redispersion/heptane precipitation procedure was repeated,preferably, twice more. A sample of 5 ml of pentane was then added tothe washed organosilane capped ZnO nanocrystals and ultrasonicated for 5minutes. The resulting mixture was then centrifuged again and theprecipitate was again collected. After discarding the supernatant, thesolid was dried in air or under vacuum, resulting in a dry whiteprecipitate which is a ZnO nanocrystalline powder.

Another method of purifying the as-synthesized organosilane capped zincoxide nanocrystals to obtain an optically clear suspension in a polarsolvent is provided. When 1.6 L of the as prepared organosilane cappedzinc oxide nanocrystal/ethanol suspension, containing >8 g ZnO, is mixedwith 3.2-4.0 L of heptane, a white precipitate forms. This whiteprecipitate is collected by centrifugation, followed by the decantingand discarding of the supernatant. A sample of 60 ml of ethanol is thenused to redisperse the white precipitate with the aid of 5 minutes ofultrasonication. 120-150 ml of heptane are used again to precipitate theproduct. After collecting the white precipitate by centrifugation,followed by decanting and discarding the supernatant for a second time,˜8 g of organosilane capped ZnO nanocrystals are obtained. To achieveeven higher purity the ethanol redispersion/heptane precipitationprocedure is repeated twice more resulting in a white precipitate.

Y₂O₃ Nanocrystals

The purification of as-synthesized Y₂O₃ nanocrystals may involve thefollowing: As-synthesized reaction mixture was precipitated withaddition of 4:1 volume percent ethanol to the reaction mixture. Thesuspension was centrifuged at 9000 rpm for 20 minutes and afterwards thesupernatant was decanted and discarded while the precipitate wascollected. This precipitate was then suspended in 2 ml of chloroform viasonication (>1 minute) and re-precipitated by the addition of 2 ml ofethanol. The suspension was centrifuged at 9000 rpm for 30 minutes,after which the supernatant was again decanted and discarded while theprecipitate was collected. The precipitate was dispersed in 3 ml ofhexane via sonication (>2 minutes) and re-precipitated with 2 ml ofethanol, where the supernatant was decanted and discarded while theprecipitate was collected. The redispersion-precipitation procedureusing hexane and ethanol was repeated once more. After this purificationprocedure, the yttrium oxide nanocrystals can be dispersed into a numberof solvents, such as chloroform, hexane, toluene and tetrahydrofuran.

The purification of the Y₂O₃ nanocrystals after the cap exchangereaction may involve the following: The nanocrystals were precipitatedwith pentane and centrifuged at 9000 rpm for 20 minutes. The precipitatewas re-dispersed in tetrahydrofuran, precipitated with hexane andcentrifuged at 9000 rpm for 20 minutes to remove the excess cappingagent and by-products. The precipitate can be dispersed into a varietyof solvents, such as tetrahydrofuran, chloroform and toluene andmixtures of solvents such as hexane and ethanol.

Example 5 Nanocomposite Formation

Formation of Nanocomposites Suspensions and Nanocomposite Layers fromCapped ZnO Nanocrystals and Polymers

Capped and purified ZnO nanocrystals, in the form of a white precipitateor nanocrystalline powders, may be dispersed in a number of polarsolvents, including, but not limited to, tetrahydrofuran, ethanol,methanol, acetonitrile, PGMEA, PGME, PGPE, ethyl lactate, cyclic ketonesand acetone, to form optically transparent suspensions. These opticallytransparent suspensions can be mixed with various polymer solutions toform uniformly dispersed ZnO/polymer nanocomposites using solventmixing. The dispersion solvent for the nanocrystals may be selectedbased on the chemical compatibility of the capping agent and thepolymer. A solvent system that is suitable for dispersing both thenanocrystals and the polymer is preferred. To form the compositesolution in the desired nanocrystal to polymer ratio, the nanocrystalsthat are dispersed in the selected solvent are mixed with a separatelyprepared solution of the polymer preferably in the same solvent or adifferent solvent, or a combination of solvents compatible with theselected solvent. These polymers include, but are not limited to, PMMA,JSR topcoat, JSR Micro (CA) brand acrylate based photoresists, Honeywellspin-on glass polymers (silicon based polymer, from Honeywell ElectronicMaterials, CA), PEO (polyethylene oxide), epoxy resins, silicones suchas Polydimethylsiloxane (PDMS) and polymethylphenylsiloxane, and epoxyresins.

An exemplary method of forming a nanocomposite suspension providesmixing a sample of 38 milligrams of purified capped ZnO nanocrystalpowder with 0.5 grams of Honeywell Electronic Material (HEM)Spin-on-Glass (SOG) polymer/ethanol solution (HW SOG, solid content is1-5% by weight). This mixture was ultrasonicated for 30 minutes,resulting in an optically transparent suspension.

Similarly, highly transparent films were obtained with epoxy or acrylicpolymers or spin-on-glasses and ZrO₂ nanocrystals with 5 nm averagesize. The nanocrystal weight loading can be varied from 0.01 to 90percent, resulting in an optically transparent suspensions and films.

A suspension of capped ZnO nanocrystals with average particle size of 3to 4 nm mixed with SOG in ethanol was used to prepare a nanocompositefilm by spin-coating the suspension on a 2 inch quartz disc at a spinrate of 500 rpm to determine the film uniformity of the resultingnanocomposite. UV-Vis spectroscopy was used to measure the opticaldensity (OD) of the film at different spots along 3 radial directions.The center of the disc was marked as 0 mm and measurements were taken at0, 3, 5, 8, 12, 16, and 20 mm from the center. The exciton peak showed amaximum at 330 nm and the deviation in the OD at 330 nm was less than2.0% for all the measurement.

The suspension of capped ZnO nanocrystals mixed with SOG in ethanol wasalso used to spin-coat a film on three 1″ quartz discs at 300, 500 and700 rpm respectively. These films were baked at 80° C. for 1 minute inair in order to remove residual ethanol. The resultant films werevisually transparent with no apparent haze or opaqueness. The nominalloading of ZnO nanocrystals in the SOG polymer nanocomposite wasmeasured to be 72.0% by weight, as calculated from the nanocompositecomposition. FIG. 11 shows the UV-Vis spectra of the resulting films.These nanocomposite films all have a band gap maximum at around 330 nmwavelength corresponding to the exciton peak of ZnO. As the spin rate atwhich the film was cast increased from 300 rpm to 700 rpm, the opticaldensity (OD) of the films decreased due to the decreasing filmthickness. The nanocomposite films are highly transparent at visiblewavelengths, as indicated by the lack of scattering above 350 nm andsharp exciton peaks in the UV-Vis spectra.

A method of forming a nanocomposite includes solvent mixing purifiedcapped zinc oxide nanocrystals of the present disclosure with PMMA intetrahydrofuran. The purified capped ZnO nanocrystals were dispersed intetrahydrofuran and then mixed with a PMMA/THF solution. FIG. 12 showsthe TEM of the nanocomposite that was spin coated on a Cu TEM grid. Thescale bar on the TEM image is 10 nm and the 4-5 nm capped ZnOnanocrystals are uniformly dispersed into the PMMA matrix withoutforming any aggregates. The inset shows a close-up of a singlenanocrystal in the nanocomposite.

The organosilane capped ZnO nanocrystals dispersed in PMMA/THF solutionwas used to prepare a nanocomposite film by spin-coating on a 2 inchsilicon wafer at a spin rate of 500 rpm. The film thickness measurementswere done by Dektak profilometer. For this measurement periodicscratches were made on the film to determine the film thickness. A 1 mmdistance was measured showing a uniform film thickness of ˜300 nm with athickness variation of <3% over this range.

Another example of a method of forming a nanocomposite of the presentdisclosure includes dispersing purified capped zinc oxide nanocrystalsof the present disclosure with an epoxy polymer in tetrahydrofuran. 500mg of as-purified organosilane capped ZnO nanocrystals were dispersedinto 2 ml tetrahydrofuran and mixed with 1.5 g epoxy, EPON™ Resin 862(Diglycidyl Ether of Bisphenol F) which is a low viscosity, liquid epoxyresin and 0.3 g Epikure™ W (Epikure W is an aromatic diamine curingagent for epoxy resin) curing agent. The mixture was transferred into amold and cured for 12 hours, and then post-cured at 150° C. for 3 hours.

Another example of a method of forming a nanocomposite of the presentdisclosure includes mixing resin EPON 862 and curing agent W (or curingagent 3295) by hand using a weight ratio of 5:1. To this mixture ZnO orZrO₂ capped with methoxytri(ethyleneoxy)propyltrimethoxysilane is thenadded. The weight ratio of the nanocrystals to the epoxy mixture can berange from 1:1000 to 10:1. A small amount of THF (no more than 200 wt %of the composite mixture) was added to reduce the viscosity of thenanocrystal/epoxy resin mixture. The mixture is then sonicated eitherinside a sonication bath or using a Hielscher UP200S sonication probefor less than five minutes. After sonication, the composite mixture (2gram to 4 grams) was then poured into an aluminum pan (4 cm diameter),which acted as a mold. The loaded pan was and placed inside a vacuumoven. Vacuum was applied in order to remove the THF and air bubbles. Theoven was then heated to 80° C. for overnight (>10 hr) under vacuum. Theresulting composite was post cured at 150° C. for another 3 hours beforeit was removed from the vacuum oven.

Another example of a method of forming a nanocomposite of the presentdisclosure may be as follows: epoxy resin EPON 862 and curing agent 3274were pre-mixed by hand using weight ratio of 10:4.3-(methacryloyloxy)propyl trimethoxysilane capped ZrO₂ nanocrystals arethen added into the epoxy resin at loading levels between 0.01-99.99 wt%. A small amount of acetone (no more than 200 wt % of the compositemixture) was added to reduce the viscosity of the nanocrystal/epoxyresin mixture. The mixture is then sonicated either inside a sonicationbath or using a Hielscher UP200S sonication probe for less than fiveminutes. The mixed composite mixture (2 gram to 4 grams) was then pouredinto an aluminum pan (4 cm diameter), which acted as a mold. The loadedpan was then placed inside a vacuum oven. Vacuum was applied to removethe acetone and air bubbles. The resulting composite was cured at roomtemperature for 24 hours before it was removed from the vacuum oven.

For spin coating 3-(methacryloyloxy)propyl trimethoxysilane cappednanoparticle/epoxy composite films, a typical protocol is described asfollows: epoxy resin EPON 862 and curing agent 3274 were pre-mixed byhand using weight ratio of 10:4. The desired amount of cappednanocryastals is then added into the epoxy resin at loading levelsbetween 1-99.99 wt %. Acetone was added to prepare a spin solution withan appropriate solid content (ranging from 10 wt % to 50 wt %). Themixture is then sonicated inside a sonication bath for 5 minutes. Thesolution can then be used directly for spin-coating. By varying thespin-rate different film thicknesses ranging from several hundrednanometers to several micrometers may be achieved.

Another example of forming a nanocomposite of the present disclosureincludes solvent mixing of purified capped zinc oxide nanocrystals ofthe present disclosure with a photoresist from JSR Micro Inc. Theas-purified capped ZnO nanocrystals were dispersed into PGMEA to form aclear suspension and JSR photoresist solution is mixed with thissuspension. The resultant suspension forms a nanocomposite film afterspin coating on a surface.

In a further example, a nanocomposite of the present disclosure isformed by solvent mixing purified capped zinc oxide nanocrystals of thepresent disclosure with a topcoat polymer from JSR Micro Inc. Theas-purified organosilane capped ZnO nanocrystals were dispersed in4-methyl-2-pentanol which was also the solvent in the JSR topcoatpolymer solution. The nanocrystal suspension was mixed with the topcoatsolution to form a dispersion which can be used to form a nanocompositefilm by spin-coating on a surface.

The method of the disclosure includes dispersing the purified cappedzinc oxide nanocrystals in water. The as-purified capped ZnOnanocrystals were dispersed into water by mixing the wet precipitate ofZnO after purification and water to form a clear suspension bysonication. This suspension was mixed with JSR aqueous topcoat solution(NFC 545-34).

In a further example, a nanocomposite of the present disclosure isformed by dispersing methoxytri(ethyleneoxy)propyltrimethoxysilanecapped HfO₂ nanocrystals in ethanol to form a suspension and mixing thissuspension with a SOG/ethanol solution. FIG. 13 shows the TEM images ofthe nanocomposite prepared by spin coating the suspension on a Cu TEMgrid. The figure inset shows a close up of the nanocrystals. Theseimages show that the 4-5 nm rice-shaped HfO₂ nanocrystals were uniformlydispersed in the SOG matrix with no visible aggregate formation.

A further example of forming a nanocomposite of the disclosure involvesdispersing methoxytri(ethyleneoxy)propyltrimethoxysilane capped ZrO₂nanocrystals of the disclosure and an acrylate based polymer in amixture of PGMEA and PGME to form a nanocomposite suspension. Films ofthis suspension are made by spin coating on quartz discs and siliconwafers. The loading of the nanocrystals in the polymer matrix is up to80 wt %. The films are made after the nanocomposite suspension isfiltered through a 200 nm filter. FIG. 14 shows the AFM image indicatingthe surface roughness of a nanocomposite film prepared by spin coatingthe suspension on a quartz disc. The Root Mean Square (RMS) roughnessvalue for this film was 0.521 nm.

In-Situ Polymerization

Nanocomposite of ZrO₂ nanocrystals and polymethyl methacrylate can beprepared by in-situ polymerization of methyl methacrylate (MMA) andnanocrystals which are at least partially capped with3-(methacryloyloxy)propyl trimethoxysilane. A typical synthesis protocolof the nanocomposite is described as follows: 500 mg MMA and 2 mg AIBNare dissolved in 9 g toluene and the solution is heated to 100° C. 0.5 gof ZrO₂ nanocrystals capped with a mixture of both3-(methacryloyloxy)propyl trimethoxysilane andmethoxytri(ethyleneoxy)propyltrimethoxysilane is dispersed in 1 g ofTHF. This dispersion is added into the MMA/toluene solution drop-wise.The mixture was maintained at 100° C. for 16 h. The reaction mixture isslightly cloudy. The resulting precipitate is collected by anti-solventprecipitation using methanol. The precipitate is then redispersed intoTHF to form a 12 wt % dispersion. Approximately 38 wt % of the solidcontent of this dispersion is from the capping agents and the PMMAaccording to thermogravimetric analysis (TGA) of the product.

Another example of nanocomposite formed by in situ polymerization ofZrO₂ nanocrystals and polymethyl methacrylate is as follows: 9 g oftoluene is heated to 100° C. 0.5 g 3-(methacryloyloxy)propyltrimethoxysilane and methoxytri(ethyleneoxy)propyl trimethoxysilanecapped ZrO₂ nanocrystals, 0.5 g MMA and 2 mg AIBN are added to 1 g THF.This mixture is added into the hot toluene drop-wise. The mixture wasmaintained at 100° C. for 16 h, after which the reaction mixture isslightly cloudy. The resulting nanocomposite is collected byanti-solvent precipitation using methanol. The precipitate is thenredispersed into THF to form a 5 wt % dispersion. Approximately 31 wt %of the solid content of this dispersion is due to the capping agents andthe PMMA according to TGA of the product.

We claim:
 1. A method of making nanocrystals comprising dissolving ormixing precursors of said nanocrystals in at least one solvent toproduce a solution, optionally at least one of heating and increasingpressure of said solution, and reacting said precursors or saidprecursors and said at least one solvent of said solution to form saidnanocrystals.
 2. The method of claim 1 wherein said nanocrystals arecapped with at least one agent to increase the solubility ordispersibility of said nanocrystals in said at least one solvent orother materials.
 3. The method of claim 2 wherein said at least oneagent comprises at least one organosilane, organocarboxylic acid ororganoalcohol.
 4. The method of claim 2 wherein said at least one agentto cap said nanocrystals is included in the solution.
 5. The method ofclaim 4 wherein said at least one agent to cap said nanocrystals iscontacted with said solution prior, during or after said reacting. 6.The method of claim 1 further comprising purifying and/or separatingsaid nanocrystals.
 7. The method of claim 6 further comprising cappingsaid purified and/or separated nanocrystals with at least one cappingagent to produce at least partially capped nanocrystals.
 8. The methodof claim 7 further comprising purifying and/or separating said at leastpartially capped nanocrystals.
 9. The method of claim 6 furthercomprising contacting said nanocrystals with a further solvent.
 10. Themethod of claim 8 further comprising contacting said at least partiallycapped nanocrystals with a further solvent.
 11. The method of claim 9wherein said further solvent includes the at least one solvent.
 12. Themethod of claim 10 wherein said further solvent includes the at leastone solvent.
 13. The method of claim 6 wherein said nanocrystals or atleast partially capped nanocrystals are dispersed in a further material.14. The method of claim 13 wherein said further material is a polymer.15. Nanocrystals or at least partially capped nanocrystals formed by amethod of claim
 1. 16. A method of forming a film or coating comprisingdispersing the nanocrystals or the at least partially cappednanocrystals of claim 15 in a further material to form a dispersion, andapplying said dispersion to a surface.
 17. The method of claim 16wherein said applying comprises spin coating, spraying, dipping, screenprinting, rolling, painting, printing, ink jet printing, depositing byevaporation and/or vapor deposition.
 18. A method of forming ananocomposite comprising combining the nanocrystals or the at leastpartially capped nanocrystals of claim 15 with a further material andforming the nanocomposite.
 19. The method of claim 1 wherein saidnanocrystals or said at least partially capped nanocrystals arecomprised of zinc oxide, hafnium oxide, zirconium oxide,hafnium-zirconium oxide, titanium-zirconium oxide and/or yttrium oxideor other semiconductor material.
 20. The method of claim 1 wherein saidat least one solvent comprises benzyl alcohol, phenol, oleyl alcohol,toluene, butanol, propanol, isopropanol, ethanol, methanol, water,propylene glycol monomethyl ether (PGME), propylene glycol methyl etheracetate (PGMEA), ethyl lactate (EL), and 2-propoxy-propanol (PnP),acetonitrile, acetone, tetrahydrofuran, cyclic ketones and mixturesthereof.